Have CSSFontFeatureValue store the tag as an AtomicString instead of a String

Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp

 /*
  * Copyright (c) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2013 BlackBerry Limited. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 509 matching lines @@
         break;
     }
 
     FontFeatureSettings* settings = description.featureSettings();
     if (!settings)
         return;
 
     unsigned numFeatures = settings->size();
     for (unsigned i = 0; i < numFeatures; ++i) {
         hb_feature_t feature;
-        String tag = settings->at(i).tag();
+        const AtomicString& tag = settings->at(i).tag();
         feature.tag = HB_TAG(tag[0], tag[1], tag[2], tag[3]);
         feature.value = settings->at(i).value();
         feature.start = 0;
         feature.end = static_cast<unsigned>(-1);
         m_features.append(feature);
     }
 }
 
 bool HarfBuzzShaper::shape(GlyphBuffer* glyphBuffer)
 {
@@ skipping 345 matching lines @@
     if (!foundToX)
         toX = m_run.rtl() ? 0 : m_totalWidth;
 
     // Using floorf() and roundf() as the same as mac port.
     if (fromX < toX)
         return FloatRect(floorf(point.x() + fromX), point.y(), roundf(toX - fromX), height);
     return FloatRect(floorf(point.x() + toX), point.y(), roundf(fromX - toX), height);
 }
 
 } // namespace WebCore